Flow meters are used to measure the mass flow rate, density, and other characteristics of flowing materials. The flowing material may comprise a liquid, gas, solids suspended in liquids or gas, or any combination thereof. Vibrating conduit sensors, such as Coriolis mass flow meters and vibrating densitometers typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness, and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flow meter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, lateral, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Density of the flow material can be determined from a frequency of a vibrational response of the flow meter. Two or more such transducers (or pick-off sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits and are typically located at positions upstream and downstream of the actuator. The two pick-off sensors are generally connected to electronic instrumentation by cabling, such as by two independent pairs of wires. The instrumentation receives signals from the two pick-off sensors and processes the signals in order to derive flow measurements.
In certain applications, the typical driver may not be feasible. This is particularly true in low flow applications where the weight of the magnets attached to the flow tubes becomes prohibitive. It is known from U.S. Pat. No. 7,168,329, for example to replace the magnets with a magnetic material applied to a portion of the flow tube itself. Such a system is adequate for simple driving frequencies such as sinusoidal or square wave using two or more drivers, i.e., one on each side of the flow tube. Recently however, the type of drive signal has become more complex than a simple square, trapezoidal, sinusoidal single-frequency drive signal. The complex drive signal may comprise two or more frequencies, for example. To implement advanced flow meter functions such as meter verification, speed of sound measurements, multiphase flow detection, etc., multiple frequencies are imposed on the flow tubes simultaneously resulting in a complex drive signal. However, in order for the flow meter to obtain meaningful information, the drive force should be both bidirectional and linear. A bidirectional drive force implies that the flow tube oscillates both towards and away from the drive assembly. A linear drive force implies that the force exerted on the flow tube is nearly linearly proportional to the current/voltage applied to the coil. Such a drive force may not be a problem in typical driver assemblies, however, in implementations such as disclosed in the '329 patent, the flow meter can only operate in either a pull-mode or a push-mode. Therefore, to obtain a bidirectional drive force, multiple drive coils are required, one on each side of the flow tube. This configuration requires an excessive number of parts, which can be costly.
In addition, the drive force should be nearly linear. Although most vibratory flow meters are built with a linear drive system, some flow meters, such as the flow meters mentioned in the '329 patent lack a linear drive signal and therefore, generally cannot support complex drive signals. One approach to address the linearity problem would be to increase the size and strength of the magnetic coil.
To address the unidirectional problem mentioned in the '329 patent, multiple coils can be used, or alternatively, a hard magnetic substance including a north/south field can be applied to the flow tube. These solutions are expensive and may be prohibitive in terms of size and power constraints.
The present invention overcomes these and other problems by incorporating a single drive coil capable of vibrating the flow tubes using a complex drive signal that may include more than one frequency.
Aspects
According to an aspect of the invention, a flow measurement system comprises:                a vibrating flow meter including:                    at least one flow tube;            a driver adapted to apply a biasing force on the flow tube; and                        a meter electronics configured to generate a drive signal to vibrate the flow tube about a first deflected position, wherein the first deflected position is offset from a flow tube rest position.        
Preferably, the drive signal includes a voltage bias.
Preferably, the biasing force applied by the driver deflects the flow tube in a first direction.
Preferably, an inherent elasticity of the flow tube deflects the flow tube in a second direction opposite the first direction.
Preferably, the drive signal vibrates the flow tube between the rest position, the first deflected position, and a second deflected position.
Preferably, the meter electronics is further configured to generate a linearization algorithm.
Preferably, the flow tube further comprises a magnetic portion.
According to another aspect of the invention, a flow measurement system comprises:                a vibrating flow meter, including:                    at least one flow tube;            a driver adapted to apply a biasing force on the flow tube; and                        a meter electronics configured to generate a voltage bias and a drive signal and apply the drive signal including the voltage bias to the driver to vibrate the flow tube.        
Preferably, the drive signal including the voltage bias vibrates the flow tube about a first deflected position with the first deflected position being offset from a flow tube rest position.
Preferably, the drive signal vibrates the flow tube between a rest position, a first deflected position, and a second deflected position.
Preferably, the biasing force applied by the driver deflects the flow tube in a first direction.
Preferably, an inherent elasticity of the flow tube deflects the flow tube in a second direction opposite the first direction.
Preferably, the meter electronics is further configured to generate a linearization algorithm.
Preferably, the flow tube further comprises a magnetic portion.
According to another aspect of the invention, a method for operating a vibrating flow meter including a flow tube and a driver comprises the step of:                vibrating the flow tube about a first deflected position, wherein the first deflected position is offset from a flow tube rest position.        
Preferably, the step of vibrating the flow tube comprises applying a first biasing force on the flow tube with a driver based on a drive signal with an inherent elasticity of the flow tube applying a second biasing force opposite the first biasing force.
Preferably, the step of vibrating the flow tube about the first deflected position comprises vibrating the flow tube between the flow tube rest position, the first deflected position, and a second deflected position, with the first deflected position between the flow tube rest position and the second deflected position.
Preferably, the method further comprises the steps of generating a drive signal including a voltage bias and applying the drive signal to the driver to vibrate the flow tube.
Preferably, the method further comprises the step of generating a linearization algorithm for a drive signal sent to the driver.